Graphene transistors on microbial cellulose

ABSTRACT

A device including a biopolymer membrane, a passivation layer on the biopolymer membrane, a graphene layer on the passivation layer, a source electrode on the graphene layer, and a drain electrode on the graphene layer, wherein the graphene layer extends between the source electrode and the drain electrode.

CROSS REFERENCE TO RELATED APPLICATIONS

This application relates to and claims the benefit of priority from U.S.Provisional Patent Application 62/118,424, filed Feb. 19, 2015 and U.S.Provisional Patent Application 62/118,828, filed Feb. 20, 2015.

STATEMENT REGARDING FEDERAL FUNDING

None

TECHNICAL FIELD

This disclosure relates to graphene transistors, and in vivo electronicsand sensors.

BACKGROUND

In the prior art, various attempts have been made at realizingbiocompatible and flexible sensors and electronics. Thin filmtransistors (TFTs) on various flexible substrates have been reportedincluding organic TFTs, Si TFTs, carbon nanotube (CNT) and graphene FETson polymer substrates, such as polyethylene terephthalate (PET),polyethylene naphthalate (PEN), and polyimide. However, the TFTs in theprior art have limited in vivo lifetime due to foreign body reactions incells, such as fibrotic encapsulation or glial scarring, and also causeinflammation.

Graphene transistors having potential application for a cellularinterface are described in “Graphene and nanowire transistors forcellular interfaces and electrical recording”, Nano Letters 10, 1098(2010). These graphene transistors are fabricated on an oxidized Sisubstrate with mechanically exfoliated graphene flakes using e-beamlithography. However, mechanically exfoliated graphene is not compatiblewith a micro-fabrication process, and it is not scalable. Further, theoxidized Si substrate described can damage tissues and nerves, which isa significant limitation against in vivo applications.

Solution-gated graphene transistors on an insulating rigid substrate aredescribed in “Graphene transistors for bioelectronics”, Proceed. of theIEEE 101, 1780 (2013). However, graphene transistors fabricated on arigid surface are not well suited for in vivo use.

Transistors formed on a microbial cellulose substrate are described in“Biocellulose based materials for organic field effect transistors”, ProEUROCON and CONFTELE 2011, Lisbon, Portugal, and “Bacterial cellulose assubstrate for inkjet printing on organic thin film transistors”, ICOE2012 Abstract. These two papers describe Pentacene- and RR-P3HT(regioregular poly(3-hexylthiophene)-based organic thin film transistors(OTFT) fabricated on a bacterial cellulose (i.e, microbial cellulose)film. Pentacene and RR-P3HT are organic semiconductors, which typicallyhave very low carrier mobility (<5 cm²/Vs). The FET mobility of thepentacene and RR-P3HT transistors on the bacterial cellulose arereported to be 0.0033 cm²/Vs, and 0.057 cm²/Vs, respectively. Due to thelow carrier mobility, the pentacene and RR-P3HT transistors need to beoperated with a high drain voltage (a few tens of volts) and a high gatepotential (a few tens of volts). The slow speed and the high dissipationpower are the major limitations to bioelectronics applications.

What is needed are biocompatible and flexible sensors and electronicsthat do not have these limitations. The embodiments of the presentdisclosure answer these and other needs.

SUMMARY

In a first embodiment disclosed herein, a device comprises a biopolymermembrane, a passivation layer on the biopolymer membrane, a graphenelayer on the passivation layer, a source electrode on the graphenelayer, and a drain electrode on the graphene layer, wherein the graphenelayer extends between the source electrode and the drain electrode.

In another embodiment disclosed herein, a method of making a devicecomprises transferring a biopolymer membrane onto a handling wafer,forming a passivation layer on the biopolymer membrane, transferringgraphene onto the passivation layer, patterning the graphene to form atleast one graphene mesa, forming a source contact on a first edge of thegraphene mesa, forming a drain contact on a second edge of the graphenemesa, etching the passivation layer surrounding the graphene mesa, thesource contact, and the drain contact to expose the bio-polymermembrane, and releasing the bio-polymer membrane from the handlingwafer.

These and other features and advantages will become further apparentfrom the detailed description and accompanying figures that follow. Inthe figures and description, numerals indicate the various features,like numerals referring to like features throughout both the drawingsand the description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show a cross sectional schematic of a biocompatiblegraphene transistor fabricated on a bio-integrative biopolymer membrane,and a scanning electron microscope (SEM) image, respectively, inaccordance with the present disclosure;

FIG. 2 shows a comparison of biocompatible and flexible transistorsshowing graphene FETs on a biopolymer membrane providing the highestcarrier mobility, g_(m) and cut-off frequency in accordance with thepresent disclosure;

FIGS. 3A, 3B and 3C show cross sectional schematics of graphene FETsfabricated on microbial cellulose (MBC) in accordance with the presentdisclosure;

FIGS. 4A, 4B, 4C, 4D, 4E and 4F show a fabrication process for graphenefield effect transistors (FETs) on a bio-polymer membrane, such asmicrobial cellulose (MBC), in accordance with the present disclosure;

FIGS. 5A, 5B, 5C, 5D, 5E and 5F show a fabrication process for agraphene-based bio-compatible sensor on a bio-polymer membrane, such asmicrobial cellulose (MBC), in accordance with the present disclosure;

FIG. 6A shows a photograph of microbial cellulose (MBC) mounted on a Sihandling wafer for processing, and FIGS. 6B and 6C show anultra-flexible MBC membrane passivated with SU-8 in accordance with thepresent disclosure;

FIGS. 7A, 7B and 7C show SEM images of microbial cellulose (MBC) mountedon a handling wafer in accordance with the present disclosure;

FIGS. 8A and 8B show optical microscope images of hippocampal neuronsgrown on MBC in accordance with the present disclosure;

FIG. 9A and FIG. 9B, which is a detailed view of FIG. 9A, show SEMimages of CVD graphene transferred on bare MBC and FIG. 9C and FIG. 9D,which is a detailed view of FIG. 9C, show MBC passivated with SU-8 inaccordance with the present disclosure;

FIG. 10 shows a sheet resistance map of CVD graphene transferred on MBCpassivated with SU-8 in accordance with the present disclosure;

FIG. 11 shows transfer (I_(ds)) and transconductance (G_(m)) curves of agraphene FET fabricated on MBC in accordance with the presentdisclosure; and

FIGS. 12A and 12B show radio frequency (RF) characteristics of agraphene FET fabricated on MBC in accordance with the presentdisclosure.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth toclearly describe various specific embodiments disclosed herein. Oneskilled in the art, however, will understand that the presently claimedinvention may be practiced without all of the specific details discussedbelow. In other instances, well known features have not been describedso as not to obscure the invention.

The present disclosure describes biocompatible graphene transistors on abio-integrative and bio-permeable membrane, microbial cellulose, forimplantable ultra-flexible and conformal sensors and electronics with along in vivo lifetime and describes related fabrication processes.

FIG. 1A shows a cross sectional schematic of a biocompatible graphenetransistor fabricated on a bio-integrative biopolymer membrane 10. Thechannel 12 between the source 14 and the drain 16 may be graphene. Agate insulator 17, which may be a dielectric is on the graphene 12 andbetween the graphene 12 and a gate 18 to insulate the gate 18 from thegraphene channel 12. The substrate 10 may be a biopolymer, such asmicrobial cellulose (MBC), and may be passivated with a polymerpassivation layer 20, which may be SU-8, polyimide, parylene, orpolydimethylsiloxane (PDMS). As further described in reference to FIGS.3A, 3B and 3C, passivation of the substrate 10 may have alternateembodiments.

FIG. 1B shows a scanning electron microscope (SEM) image of a transistorfabricated on a biopolymer membrane 10 in accordance with the presentdisclosure.

The present disclosure describes the utilization of microbial celluloseas a substrate for biocompatible graphene transistors. Neurons can growin such a substrate. Also described is a scalable microfabricationprocess for fabricating graphene transistors on a microbial cellulosemembrane, which is fully compatible with conventional semiconductordevice fabrication technologies. Use of a microbial cellulose membranehas a number of benefits including cell attachment and proliferation, noimmunological reactions in the body, biocompatibility andchemical/mechanical stability in in vivo environments. Microbialcellulose has been previously used in the prior art as a tissueregeneration scaffold. In addition, microbial cellulose is commerciallyavailable at a low cost in a large scale in sheets and rolls, which isbeneficial for developing large-scale fabrication processes compatiblewith roll-to-roll processes.

For in vivo medical diagnoses/monitoring, neural prosthetics, andbrain-machine interfaces, significant research efforts have beenexpended to develop biocompatible and implantable sensors andelectronics. In the prior art, biocompatibility of implantable sensorsand electronics is largely attained by coating the sensors andelectronics with biocompatible polymers, for example polyimide,polydimethylsiloxane (PDMS), and/or parylene. However, the delaminationor cracking of polymer coatings results in limited in vivo lifetimes ofthe sensor probes and electronics. Further, the devices and probesfabricated with standard semiconductor micro fabrication technology arestructurally rigid, and are invasive and incongruent to biologicalsystems. Also, polymer-based flexible semiconductor devices in the priorart have a low carrier mobility (<80 cm²/Vs), which results in reducedsignal-to-noise ratio.

To address these limitations of the prior art implantable sensors andelectronics, the present disclosure describes graphene FETs fabricatedon a biocompatible and ultra-flexible membrane for in vivo sensor andelectronics applications. As described in the present disclosure,chemical vapor deposition (CVD)-grown graphene transferred onto anultra-flexible biopolymer membrane provides high carrier mobility(>1,600 cm²/Vs), high flexibility of up to 18% strain, and chemicalstability suitable for in vivo conditions. The bio-membrane substratecan serve a scaffold for neuron/tissue regrowth, which is highlybeneficial for realizing implantable devices with a long (>a few years)in vivo lifetime.

FIG. 2 shows a comparison of prior art biocompatible and flexibletransistors with the graphene FETs on a biopolymer membrane of thepresent disclosure. As shown in FIG. 2, graphene FETs on microbialcellulose (MBC) in accordance with the present disclosure have thehighest sensitivity (g_(m)) and the highest cut-off frequency or speedcompared to prior art pentacene TFTs andPoly(3,4-ethylenedioxythiophene)(PEDOT) TFTs.

FIGS. 3A, 3B and 3C show cross sections of alternate embodiments ofgraphene FETs fabricated on microbial cellulose (MBC) in accordance withthe present disclosure. As shown in FIG. 3A, the MBC 10 may bepassivated with a dielectric film 30, such as Al₂O₃, SiO₂, or HfO₂ grownon the MBC 10 using atomic layer deposition (ALD). In an alternateembodiment, as shown in FIG. 3B, a dielectric such as Al₂O₃, SiO₂, orHfO₂ layer 30 may be grown on the MBC 10 using atomic layer deposition(ALD), and then a bio-compatible polymer 34, such as SU-8, polyimide,parylene, or PDMS, is deposited on the dielectric. In yet anotherembodiment, as shown in FIG. 3C, a bio-compatible polymer 36, such asSU-8, polyimide, parylene, or PDMS, is used as a passivation layer onthe MBC 10. In FIGS. 3A, 3B and 3C, for sensor applications, the gatedielectric 17 and gate electrode 18 are optional and may not be present,as for a sensor they may not be needed.

FIGS. 4A, 4B, 4C, 4D, 4E and 4F show a fabrication process for graphenefield effect transistors (FETs) on a bio-polymer membrane, such asmicrobial cellulose (MBC), in accordance with the present disclosure. Asshown in FIG. 4A, a biopolymer 10, which may be a MBC membrane, istransferred on a handling wafer 40, which may be a common semiconductoror an insulator, such as Si, Ge, InP, GaAs, SiO₂, sapphire, quartz, orglass, and then dried in air. Then a passivation layer 42, which may bean ALD grown dielectric, such as an Al₂O₃, SiO₂, or HfO₂ layer, or abio-compatible polymer such as SU-8, polyimide, parylene, or PDMS or acombination of both is applied over the MBC film 10. The thickness ofthe MBC membrane 10 may be less than 10 μm, the thickness of thepassivation layer may be less than 10 nm for an ALD dielectric, andapproximately 1 μm for a bio-compatible polymer. Next, as shown in FIG.4B, a CVD graphene film 44 is transferred over the passivated MBCmembrane. Next, as shown in FIG. 4C, the transferred graphene 44 may bepatterned using a lithography methods well known in the art and etchedwith oxygen plasma to form graphene mesa structures 46, and to exposethe passivation layer 42 surrounding the graphene mesa structures 46.

Then, as shown in FIG. 4D, an ohmic source 48 contact and an ohmic draincontact 50 are formed on edges of a graphene mesa 46. Next, as shown inFIG. 4E, a gate dielectric 17 is formed on the graphene mesa structure46 and then a gate electrode 18 is formed on the gate dielectric 17 tocomplete a graphene FET. Then, as shown in FIG. 4F, the passivationlayer 42 surrounding the graphene FET can be etched away from thebackground exposing the bio-polymer membrane 10, which as described maybe an MBC membrane. Finally the graphene FET and the MBC membrane may bereleased or demounted from the handling wafer 40.

FIGS. 5A, 5B, 5C, 5D, 5E and 5F show a fabrication process for agraphene-based bio-compatible sensor on a bio-polymer membrane, such asmicrobial cellulose (MBC), in accordance with the present disclosure.The process is similar to that described in reference to FIGS. 4A, 4B,4C, 4D, 4E and 4F; however, for sensor applications, integration of agate dielectric and a gate electrode is not needed.

As shown in FIG. 5A, a MBC membrane 10 is transferred on a handlingwafer 40, which may be a common semiconductor or an insulator, such asSi, Ge, InP, GaAs, SiO₂, sapphire, quartz, or glass, and then dried inair. Then a passivation layer 42, which may be an ALD grown dielectric,such as an Al₂O₃, SiO₂, or HfO₂ layer, or a bio-compatible polymer suchas SU-8, polyimide, parylene, or PDMS or a combination of both isapplied over the MBC film 10. The thickness of each layer is usuallyless than 10 μm for the MBC membrane 10, less than 10 nm for an ALDdielectric, and approximately 1 μm for a bio-compatible polymer. Next,as shown in FIG. 5B, a CVD graphene film 44 is transferred over thepassivated MBC membrane. Next, as shown in FIG. 5C, the transferredgraphene 44 may be patterned using a lithography methods well known inthe art and etched with oxygen plasma to form graphene mesa structures46, and to expose the passivation layer 42 surrounding the graphene mesastructures 46.

Then, as shown in FIG. 5D, an ohmic source 48 contact and an ohmic draincontact 50 are formed on edges of a graphene mesa 46. Next, as shown inFIG. 5E, the passivation layer 42 surrounding the graphene-basedbio-compatible sensor can be etched away from the background exposingthe bio-polymer membrane 10, which as described may be an MBC membrane.Finally, as shown in FIG. 5F, the graphene-based bio-compatible sensorand the MBC membrane may be released or demounted from the handlingwafer 40.

FIG. 6A shows a photograph of microbial cellulose (MBC) 10 mounted on aSi handling wafer 40 for processing, and FIGS. 6B and 6C show anultra-flexible MBC membrane 10 passivated with a biocompatible polymer,namely SU-8.

FIGS. 7A, 7B and 7C show SEM images of microbial cellulose (MBC) mountedon a Si handling wafer in accordance with the present disclosure. MBCconsists of pure crystalline nano-fibrils, which have diameters of lessthan 50 nm and lengths of tens of μm.

FIGS. 8A and 8B show optical microscope images of hippocampal neurons 60grown on MBC. These images show that MBC is biocompatible and indicatesthe bio-integrativity of MBC.

FIG. 9A and FIG. 9B, which is a detailed view of FIG. 9A, show SEMimages of chemical vapor deposition (CVD)-grown graphene transferred onbare MBC. FIG. 9C and FIG. 9D, which is a detailed view of FIG. 9C, showMBC passivated with SU-8. The transferred graphene, shown in FIGS. 9Aand 9B, is continuous across the entire sample and shows nearly nodefects in the graphene caused by the transfer process. Minor defects inthe images originate from growth defects in the CVD graphene. The sheetresistance and carrier mobility of the transferred graphene has beenmeasured to be in the range of 450-1100 ohm/sq and 1600-2050 cm²/Vs,respectively.

FIG. 10 shows a sheet resistance map of CVD graphene transferred on MBCpassivated with SU-8. The average sheet resistance R, is approximately590 Ω/sq.

FIG. 11 shows the transfer (I_(ds)) and transconductance (G_(m)) curves,respectively, for a graphene FET fabricated on MBC. The contactresistance of ohmic contacts measured using transmission linemeasurement (TLM) patterns is less than 0.1 Ωmm, which is one of thelowest contact resistance values that has been reported in the prior artfor graphene FETs. A high G_(m) of 204 mS/mm is measured at V_(ds)=1Vfor a graphene FET with L_(g)=2 μm, L_(sd)=2.5 μm, W=12 μm. The highG_(m) enables high sensitivity for sensing.

The radio frequency (RF) characteristics of a graphene FET on MBC areshown in FIGS. 12A and 12B. A maximum F_(T) of 2.5 GHz and a maximumF_(max) of 4.2 GHz have been measured for a graphene FET with L_(q)=1.5μm, L_(ds)=2 μm, W_(q)=2×50 μm, which demonstrates that the graphene FETon MBC in accordance with the present disclosure operates in the GHzrange.

The present disclosure has described graphene FETs fabricated on abiopolymer, such as microbial cellulose (MBC) and fabrication processesof graphene FETs on MBC and graphene sensors on MBC. These FETs andsensors may be used for in vivo electronics and sensor applications. Useof a biopolymer, such as MBC, provides a tissue/neuron-regenerativesubstrate for biosensors and bioelectronics, as demonstrated in FIGS. 8Aand 8B. The used of CVD graphene and biopolymer substrates enablesflexible and stretchable electronics and sensors for in vivoapplications, such as a personal wireless body network.

Having now described the invention in accordance with the requirementsof the patent statutes, those skilled in this art will understand how tomake changes and modifications to the present invention to meet theirspecific requirements or conditions. Such changes and modifications maybe made without departing from the scope and spirit of the invention asdisclosed herein.

The foregoing Detailed Description of exemplary and preferredembodiments is presented for purposes of illustration and disclosure inaccordance with the requirements of the law. It is not intended to beexhaustive nor to limit the invention to the precise form(s) described,but only to enable others skilled in the art to understand how theinvention may be suited for a particular use or implementation. Thepossibility of modifications and variations will be apparent topractitioners skilled in the art. No limitation is intended by thedescription of exemplary embodiments which may have included tolerances,feature dimensions, specific operating conditions, engineeringspecifications, or the like, and which may vary between implementationsor with changes to the state of the art, and no limitation should beimplied therefrom. Applicant has made this disclosure with respect tothe current state of the art, but also contemplates advancements andthat adaptations in the future may take into consideration of thoseadvancements, namely in accordance with the then current state of theart. It is intended that the scope of the invention be defined by theClaims as written and equivalents as applicable. Reference to a claimelement in the singular is not intended to mean “one and only one”unless explicitly so stated. Moreover, no element, component, nor methodor process step in this disclosure is intended to be dedicated to thepublic regardless of whether the element, component, or step isexplicitly recited in the Claims. No claim element herein is to beconstrued under the provisions of 35 U.S.C. Sec. 112, sixth paragraph,unless the element is expressly recited using the phrase “means for . .. ” and no method or process step herein is to be construed under thoseprovisions unless the step, or steps, are expressly recited using thephrase “comprising the step(s) of . . . .”

What is claimed is:
 1. A device comprising: a biopolymer membranecomprising a biopolymer; a passivation layer on the biopolymer membrane;a graphene layer on the passivation layer; a source electrode on thegraphene layer; and a drain electrode on the graphene layer; wherein thegraphene layer extends between the source electrode and the drainelectrode; wherein a thickness of the biopolymer membrane is less than10 μm; and wherein the biopolymer membrane comprises microbialcellulose.
 2. The device of claim 1 wherein: the source electrodeextends over the passivation layer; and the drain electrode extends overthe passivation layer.
 3. The device of claim 1 wherein the passivationlayer comprises a dielectric.
 4. The device of claim 1 wherein thepassivation layer comprises: a dielectric on the biopolymer membrane;and a bio-compatible polymer on the dielectric.
 5. The device of claim4: wherein the dielectric comprises Al₂O₃, SiO₂, or HfO₂; and whereinthe bio-compatible polymer comprises SU-8, polyimide, parylene, orpolydimethylsiloxane (PDMS).
 6. The device of claim 1 wherein thepassivation layer comprises a bio-compatible polymer on the biopolymermembrane.
 7. The device of claim 6 wherein the bio-compatible polymercomprises SU-8, polyimide, parylene, or PDMS.
 8. The device of claim 1wherein the graphene layer is a chemical vapor deposition-growngraphene.
 9. The device of claim 1 further comprising: a gate insulatoron the graphene layer; and a gate electrode on the gate insulator;wherein the gate electrode is between the source electrode and the drainelectrode.
 10. The device of claim 9 wherein the gate insulatorcomprises a dielectric.
 11. The device of claim 1 wherein the device isa sensor or a transistor.
 12. The device of claim 1: wherein a carriermobility is greater than 1,600 cm²/Vs; and wherein a flexibility is upto 18% strain.